(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same and, more particularly, to a negative active material for a rechargeable lithium battery which is doped with boron.
(b) Description of the Related Art
Metallic lithium has been used for negative active material in a rechargeable lithium battery. However, lithium metal has good reactivity toward electrolyte and deposits to form a dendric layer which increases the reactivity of lithium. There are at least two disadvantageous effects that arise from the reaction of lithium with electrolyte: the exothermic liberation of heat and the formation of passive films on lithium has been shown to be one reason for the loss of capacity of lithium cells on repeated cycling.
The problem of lithium reactivity toward the electrolyte is addressed by replacing lithium metal with carbon-based materials. With the use of carbon-based active materials, the potential safety problem present in metallic lithium-based batteries can be prevented while achieving a relatively higher energy density as well as the reasonable shelf life.
The carbon-based materials can be largely classified into two categories crystalline graphite and amorphous carbon. Among the crystalline carbon-based material, artificial graphite has the desired properties such as ability to control density and interlayer spacing by adjusting the temperature of the heat treatment. Artificial graphite is prepared by heat-treating petroleum pitch, coal pitch, polymer-based pitch or soft carbon at 2600-3000xc2x0 C. Alternatively, doping with boron as catalyst may increase the capacity of coke type materials to a suitable degree. Several methods are disclosed to increase the catalyst content in the resulting graphite structure since the catalyst content acts as a critical factor in obtaining the desired crystalline structure.
Japanese Patent Laid-open No. 9-63584, 9-63585 and 8-306359 disclose methods in which a boron compound is doped in carbon fiber or graphite, after which the boron-doped carbon fiber or graphite is heated. Although these methods are simple, the boron remains on the surface of the carbon fiber or graphite because boron is directly mixed with carbon fiber or graphite without the use of any solvent. Accordingly, boron is not introduced into the graphite but rather is physically mixed with graphite. To increase the content of boron relative to the carbon-based material, the size of carbon-based material and that of the boron should be decreased. However, small carbon-based materials, particularly carbon fiber can used for an active material. That is, there is a limit to the decrease in the size of carbon-based material.
Japanese Patent Laid-Open No. 5-26680 discloses a method in which boric acid, used as boron compound, is added to petroleum pitch and the mixture is heat-treated. Using the method, boron is better introduced into graphite without the use of a solvent. That is, a wet method is utilized in this process, whereas mixing method without a solvent is referred to as a dry method. However, it is still necessary in this method to increase the content of boron in the carbonaceous material.
There is also proposed a method in which a gaseous low molecular carbon-based materials react with a gaseous boron-based compound by using a CVD (chemical Vapor Deposition) process. The CVD process can dope boron in the graphite, but the method is complicated and the production costs are high.
It is an object of the present invention to provide a negative active material for a rechargeable lithium battery having a high content of boron which is uniformly doped in the negative active material.
It is another object to provide a method of preparing a negative active material for a rechargeable lithium battery which is simple and inexpensive to produce.
These and other objects may be achieved by a negative active material for a rechargeable lithium battery and a method of preparing the same. The method includes the steps of preparing a mixed solution including a pitch, a boron-based compound and a solvent, removing the solvent from the mixed solution, heat-treating the resulting mixture to produce a mesophase pitch, carbonizing the mesophase pitch and graphitizing the carbonized mesophase pitch.
The negative active material includes 0.1 to 10 wt % of boron. The negative active material has an planar distance of d002xe2x89xa63.40 xc3x85 of X-ray diffraction plane distance at the (002) plane and a crystallinity size in the direction of the c axis of Lcxe2x89xa7400 xc3x85. The present invention further provides a negative active material for a rechargeable lithium battery including shapeless graphite, which has an edge rolled with an onion sheath structure such that an interlayer spacing of the graphite is closed.